1. Field of the Invention
The present invention relates to an image sensor and a photoelectric conversion apparatus using the same and, more particularly, to an image sensor having a plurality of light-receiving elements, and a photoelectric conversion apparatus having a plurality of image sensors each having an array of a plurality of light-receiving elements.
2. Related Background Art
In recent years, various types of sensors have been commercially available along with developments of semiconductor technology. One of these sensors extracts an electrical signal from each photosensor portion and performs image processing.
Conventional linear image sensors are very popular in facsimile machines, scanners, and digital copying machines. Such a linear image sensor is formed on a silicon wafer, and the sensor length is limited by the wafer size. It is difficult to form one image sensor having an array of a plurality of light-receiving elements, the overall length of which is equal to the length of an original. For this reason, an image on the original is reduced by directing light reflected by the original on an optical system. A reduced image is projected on the linear image sensor, thus reading the original image.
A reader having the reduction optical system described above requires a large space for the optical system. In addition, it is difficult to obtain a sufficiently high resolution in this reader.
In order to solve this problem, a multi-chip image sensor having a one-dimensional array of a plurality of linear image sensors is used.
FIG. 1 is a view illustrating a circuit arrangement of a conventional image sensor.
As shown in FIG. 1, light signals from light-receiving elements 6-1 to 6-n are selected by a control circuit 3 in synchronism with fundamental clock pulses and are sequentially read and output to a common output line 1.
A signal output onto the output line 1 is input to a light signal amplifier 4 through a reset circuit 5. The reset circuit 5 alternately performs resetting and read access. A signal amplified by the light signal amplifier 4 is output onto a common output line 2.
FIG. 2A is a circuit diagram showing a detailed arrangement of the reset circuit. FIG. 2B is a timing chart of waveforms of signals for driving the reset circuit and a waveform of an output signal from the reset circuit.
Referring to FIG. 2A, a transistor QS is a transistor for reading out a pixel signal Vo from the output line 1 to an output line 1a as an output signal Vout. A transistor QR is a transistor for resetting the output line 1. A control signal .phi.A is input to the gate electrode of the transistor QS, and a control signal .phi.B independent of the control signal .phi.A is input to the gate electrode of the transistor QR.
As shown in FIG. 2B, during a time interval t1 as an ON time of the signal .phi.A, a pixel signal from the output line 1 is read and output to the output line 2 serving as an input line of the amplifier 4. During a time interval t2 as the ON time of the signal .phi.B, the output line 1 is reset. In this manner, read access and resetting are alternately performed, and a duty ratio of the output signal becomes about 50%.
In the conventional example, however, since the pixel signals of each image sensor are synchronized with fundamental clock pulses, and read access and resetting are serially performed, the read signal has a maximum duty ratio of 50% or less. It is difficult to cope with fundamental clocks having higher frequencies.
When the number of pixels is increased and the sensor length is increased to obtain a higher resolution, a parasitic capacitance of a horizontal read line for reading a signal is increased, and read access of the light signal cannot be performed at a sufficiently high speed. The signal level is decreased, and an S/N ratio is also decreased. In order to solve these problems, multi-line read access is developed and used in some fields of practical applications.
FIG. 3 is a schematic view showing an arrangement of a multi-line read system.
Referring to FIG. 3, a one-line sensor 301 is divided into a plurality of blocks, and signals of the sensor portions in each block are read and output onto a corresponding horizontal read line 302. Signals on the horizontal read line 302 are transferred to an output amplifier 304 through a switching means 303, and output signals are extracted from an output terminal 305.
In a multi-line read system, when the number of sensors for one line is increased, the number of blocks and the number of horizontal read lines must be increased, resulting in a complex circuit arrangement.